The present invention relates generally to a method of improving the phase relationship between optical and electrical scans in a scrolling color projector and, more particularly, to a method of restoring a uniform phase relation between the optical and electrical scans.
In scrolling color projection systems an arc lamp is frequently utilized as a source of an intense white light which is collected and separated using dichroic filters into the primary colors red, green and blue (RGB). This color separated light is caused to be formed into three sources, arrayed adjacently, such that each source appears to be narrow in a vertical direction and wider in a horizontal direction. Scanning optics are employed to cause these three bands of light, one of each of the primary colors, to be positioned onto the rear of a transmissive light valve panel. This panel may be a twisted nematic liquid crystal display (LCD) panel with a thin film transistor (TFT) addressing or other suitable types of light valves. The scanning optics causes the bands of primary color illumination to move sequentially across the LCD panel. As a band passes over the xe2x80x9ctopxe2x80x9d of the active area of the LCD panel, a band of light of that color again appears at the xe2x80x9cbottomxe2x80x9d of the panel. Accordingly, there is a continuous sweep of the three primary colors (RGB) across the panel.
Prior to each color passing over a given row of pixels on the LCD panel, that row will have been addressed with the appropriate information for that color. This means that each row of the panel will be addressed three times for each video field which is to be displayed. This can be accomplished by either using extra addressing lines to the panel array, and writing the horizontal rows in parallel, or by writing three separated rows sequentially, but at three times the field rate. The information being written to the separated rows must be appropriate for the color content of that portion of the image which is being displayed.
Light from the LCD panel is projected by a color corrected projection lens or, alternatively, used in a direct view mode. The simultaneous use of a large portion of the available red, green and blue light to a single light valve panel eliminates the need to mechanically converge the image, and further reduces system costs. However, the speed of the color stripes produced by the prism scanner used in scrolling color projectors is not constant, but depends on the location of the color stripe on the LCD panel. Therefore, the electrical scan of the LCD panel, which occurs at constant speed, cannot be everywhere in phase with the illumination. This non-uniformity of phase relation between the optical and electrical scan causes color cross talk that varies over the panel.
Accordingly, the present invention is directed to overcoming one or more of the problems or disadvantages associated with the relevant technology.
In a preferred embodiment of the method of this invention, the addressing sequence of the LCD panel is altered to change the rate of any one color relative to the others. Such a change in sequence can momentarily increase the speed of one channel while correspondingly decreasing the speed of the other two channels. Depending upon the speed variations necessary, only occasional phase adjustments of the addressing sequence need be made.